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The difference is between static and dynamic IP addresses. Static IPs are constant and do not change for a given account. Dynamic IPs are assigned upon logging in.
Unless there's something I am unaware of, an IP does not change within a given session, but should a person with dynamic IP disconnect and reconnect, it can change, although it will still be within the set range for that ISP.
Static IPs are becoming mroe common, but dynamic IPs are still often used. Even my DSL is dynamic. Static is available, and I could have one if I wanted, but I have no specific need for it so I haven't asked.
All that being said however, there are a few programs as well as paid services that will route internet traffic through a series of global servers called proxies and essentially mask a user's true IP.
Now someone with the appropriate resources and knowledge could actually retrace the routings and potentially be able to discover the originating IP, but for the average Joe like me or you we wouldn't have a clue.
This is my understanding of the subject and I haven't had a need to really learn the full details of the process, so others will definately have something to add and possibly corrections to what I have said.
I would be interested in becoming better versed on this topic myself so I will be anxious to read what others have to say as well.
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